X
X-rays
X-rays were discovered accidentally by Wilhelm Röntgen in 1895. X-ray radiation is a part of the electromagnetic spectrum. With wavelengths ranging from about 0.001nm to about 100 nm, X-rays are much shorter than ultraviolet. They travel at the speed of light.
Z
Zenith
The zenith is the point in the sky directly above an observer's head. This term is often used in astronomy.
Zeroth law of thermodynamics
If body A is in thermal equilibrium with body B, and B is also in thermal equilibrium with C, then A is necessarily in thermal equilibrium with C.
This is equivalent to saying that thermal equilibrium obeys a transitive mathematical relation.
Z boson
A carrier particle of weak interactions. It is involved in all weak processes that do not change flavor.